An optical grating is a critical element in optics especially for the systems where specific wavelength dependent functions are performed. For example, a spectrometer is an instrument to measure the wavelength of the incident light and the spectral width of the light. A simple spectrometer diagram 100 is illustrated in FIG. 1. As shown in FIG. 1, incident light from a light source 102 is reflected by a mirror 104 and then diffracted by a grating 106. Light diffracted from the grating 106 is reflected by a second mirror 108. Reflected light is then collected by a detector 110.
The theory of a grating operation can be found in many textbooks. There are many different types of gratings. A reflection blazed grating 106 is illustrated in FIG. 1. The diffraction effect is produced by having periodic structure either inside the media or on the surface of the substrate. In general, as shown in FIG. 2, depending on the grating surface profile, there are multiple orders of the diffraction.
The well-known grating equation is given below:Λ(sin θin+sin θN)=Nλ  Equation 1where
N=0,±2, . . . if solutions exist.
λ: wavelength of light
θin: the angle of incidence beam
θN: the diffraction angle of Nth order and,
Λ: the periodicity of the grating
For each diffraction order, the diffraction angle θN is a function of wavelength as depicted in Equation 1. In addition, the diffraction efficiency is a function of the polarization of the input light. As shown in FIG. 3, the responses for S- and P-polarizations are different. The difference of the losses between these two polarizations is known as polarization dependent loss (PDL).
The dispersion coefficient can be computed by taking derivatives of both sides of Equation 1;
                                          ⅆ                          θ              N                                            ⅆ            λ                          =                  N                      Λcos            ⁢                                                  ⁢                          θ              N                                                          Equation        ⁢                                  ⁢        2            
The dispersion coefficient is higher for higher order modes. However, the diffraction efficiency will decrease as the number of allowed higher order modes increases. This represents a dilemma of the grating design.
The grating response is also temperature dependent because of change of the index of refraction of the grating material with temperature and/or changes in the periodicity of the grating structure due to thermal expansion.
Also, by carefully selecting the periodicity A of the grating, shown in Equation 1, it is possible to reduce the number of allowed diffraction orders to only a single order to avoid loss and cross talk.
A diffraction grating is commonly used to cause dispersive response to the input optical beam. The exit beam angle varies with the wavelengths. Even with the advancement of grating fabrication technology, the tradeoff between the diffraction efficiency (DE), dispersion coefficient (dθ/dλ) and its associated polarization dependent loss (PDL) remain an important design issue, in particular, when a very high dispersion coefficient is needed.
U.S. Pat. No. 4,025,196 disclosures an apparatus to perform correlation spectroscopy utilizing a zero dispersion monochromator having entrance, intermediate and exit slits. A ruled grating is located in the beam path between entrance and exit slits and is operable to disperse a beam of radiation incident thereon both prior and subsequent to radiation passage through the intermediate slit.
U.S. Pat. No. 5,652,681 disclosures a dispersive optical element called a “grism” which has characteristics of both a prism and a grating. The grism consists of a prism with a grating disposed adjacent to one surface of the prism. Light passing through the grism is dispersed by both the prism and the grating. The grating may be attached to either the first or second surface of the prism or may be simply adjacent to the prism. The grism has dispersive characteristics such as resolving power that can be optimized in a very flexible manner by choice of both the grating and prism characteristics. For example, the grating may be used to amplify the angular spread introduced by a prism. Also different diffractive orders of the grating may be used simultaneously.
It is possible to use two gratings in tandem to enlarge the overall dispersion coefficient. Such a configuration, used in spectroscopy, is called a double monochromator as described in “Diffraction Grating Handbook,” fourth edition, Christopher Palmer, Richardson Grating Laboratory, Copyright 2000, pg. 55. In addition, one can insert a half-wave plate (HWP) between these two gratings to compensate the PDL.
A high performance diffraction grating is relatively expensive thus it is desired not to use two gratings if possible. The concept of double passing a single grating is known in the art. For instance, Kaiser Optical System, Inc. Ann Arbor, Mich. discusses the use of double passing a single transmission volume phase grating with a single reflective mirror to double the wavelength angular dispersion (http://www.kosi.com/). Similarly Wasatch Photonics, Inc., of Logan, Utah discusses double passing a transmission grating with a single reflective mirror for increased dispersion (http://www.wasatchphotonics.com/) In U.S. Pat. No. 6,563,977, a single grating configuration was also proposed, that is double passed for increasing the dispersion, to be used in WDM (wavelength division multiplexing) modules as shown in FIG. 4.
FIG. 4 shows a block diagram generally illustrating a multiplexer-demultiplexer device 40. The device 40 includes an array 42 of optical fibers 4, each of the fibers 4 being either a multi-mode fiber or a single mode fiber; a single fiber 43 that is either a multi-mode fiber or a single mode fiber; a transmissive grating assembly 44 having a diffractive element for diffracting beams propagating therethrough; a polarization rotating element 46 for rotating the polarization plane of beams passing therethrough; a first focusing and collimating lens 48 for focusing and collimating beams propagating between the ends of the array 42 of fibers and the grating assembly 44, the first lens having a focal length associated therewith; a second focusing and collimating lens 50 for focusing and collimating beams propagating between the end of the single fiber 43 and the grating assembly 44, the second lens having a second focal length associated therewith; a first mirror 52 for reflecting beams radiating between the array 42 of fibers and the polarization rotating element 46 via the grating assembly 44; and a second mirror 54 for reflecting beams radiating between the single fiber 43 and the polarization rotating element 46 via the grating assembly 44. The end of the fiber 43 is located at the vicinity of the focal point of the collimating lens 50 which is formed from at least one piece of optical glass.
However, also illustrated in FIG. 4, the physical separation of the optical beam paths is necessary to make room for the half-wave plate. This greatly increases the optical path length, making a compact design difficult, adding to the design challenges and also degrades the optical performance. The temperature dependence of the grating is also an important issue that is neglected in this design which especially important when three elements need to be actively aligned and fixed and held over operational temperature conditions with small tolerance. Small changes in these components lead to angular deviations of the light that reduce the output coupling of the device. In addition, what was not investigated or prescribed how to achieve in U.S. Pat. No. 6,563,977 is that there are many side effects due to multiple reflections and undiffracted (zeroth order) and higher order diffraction modes making the idea in U.S. Pat. No. 6,563,977 impractical. This is especially true for applications requiring high signal to noise ratios. Because of the interference of these reflections and multiply diffracted beams with the primary diffraction the design in U.S. Pat. No. 6,563,977 cannot support the low noise required for WDM, or the requirements for low insertion loss ripple and low multipath interference. These considerations are especially important in WDM applications where signal-to-noise ratios need to be 40 dB or greater.
Thus, we find that double passing a grating is known in the art but several important and essential operational features are not addressed in the art. For instance, U.S. Pat. No. 6,563,977 does not prescribe how to make the optical assembly thermally stable with low pointing error versus temperature, does not prescribe how to make the optical assembly produce high signal to noise ratios, does not prescribe how to achieve low insertion low ripple, does not prescribe how to achieve low back reflection, and does not prescribe how to reduce or eliminate multipath interference. All of these aforementioned qualities must be addressed in the design concept to make the use of double passing a transmission grating for WDM or DWDM applications practical.
It is within this context that embodiments of the present invention arise.